What Causes Gradual Reduction In Platelet Count?

hi, i have low platelets ct of 97,000. iwas able to check my cbc results and noticed everything has been normal for the last 5 yrs.,except a slow decline of the platelet ct. i have a titanium knee replacement approx. 10 yrs.old i am diabetic and take metformin, juvenia,glyburide. i also take levothroid and simvastain. i do not bruise easily and don t have bleeding issues. i recently have been getting head colds and with that occas. resp. infections approx. q 3-6 months. any ideas. why my platelets are low.